The present invention relates to methods and apparatus for enhancing the covertness of a spread spectrum transmission and in particular to auto-balance methods and apparatus for suppression of residual lines in a signal spectrum.
In certain covert communication systems it is desirable to spread a signal into a wide band that looks like noise. Such spreading can be accomplished by the use of a bi-phase modulator which splits a carrier signal into segments and alters the phase of each segment in response to a binary signal received from a pseudo-random bit sequence code generator. The response to the bit sequence is such that a logic zero level bit allows an in-phase carrier signal and the logic 1 level bit changes the phase of the carrier signal by 180.degree.. The greater the frequency with which bits are supplied by the binary code generator, called the code clock frequency, the greater is the spreading of signal frequency that results.
Where covertness is the primary objective of an RF link, the practical limit on the obtainable covertness of a transmitter is determined by the degree of suppression of residual lines in the transmitted spectrum. Residual lines are caused by continuous signals, such as the carrier and the carrier.+-.the code clock frequencies. These lines are, in general, more detectible by narrow band spectrum analysis techniques than are the transitory lines in the spectrum associated with the spreading process as long as an adequately designed spreading function is employed.
If all the balances in the transmitter are carefully made, including the balance in the spreading modulator, the balance of rise and fall shapes and times in the spreading function (code) and the balance of the one and zero energy of the spreading function, then the residual lines are probably suppressed to a level that provides the desired covertness. Nevertheless, although those balances associated with the design of the spreading function can be relied upon to remain as designed, those adjustments associated with modulator balance are subject to drift and change due to temperature, time and other factors. At microwave frequencies, when the proper adjustments are made, the most critical spectrum line, the suppressed carrier, can easily be suppressed below the unmodulated carrier by at least 20 db and can with a great deal of care and design difficulty (e.g., by employing temperature control) be held to 40 db below the unmodulated carrier.
Because of the difficulty associated with providing the appropriate transmitter balances, one approach to the problem of residual lines involves nulling the residual carrier component by employing phase and amplitude tracking circuits. However, such circuits are costly and complex so that the implementation, the fabrication and the testing of such circuits is difficult. Furthermore, such carrier nulling does not operate on residual continuous wave spectral components other than the carrier.
Therefore, it is desirable to have a simple means for enhancing the covertness of a spread spectrum transmission in order to make detection by undesirable or unintentional receivers difficult, if not impossible.